In GSM, packet data services with high peak rates are provided by means of EDGE technology. EDGE introduces a set of modulation and coding schemes (MCS). The different schemes result in different radio link bit rates and robustness, and are therefore optimal for use in different link quality regions (e.g. C/I). Link Quality Control (LQC), comprising link adaptation and hybrid ARQ (Incremental Redundancy, IR) algorithms, ensures that the most efficient scheme is always used for a mobile by means of quality information feedback from the mobile to the system, see Furuskär, S. Mazur, F. Müller and H. Olofsson, “EDGE: Enhanced data rates for GSM and TDMA/136 evolution”, IEEE Pers. Commun., vol. 6, pp. 56-66, June 1999. Typically the throughput is maximized or the packet delay is minimized (i.e., the number of retransmissions is minimized by increased coding and robustness) by the algorithms.
To further increase the EDGE packet data peak rate, dual carrier operation is currently being introduced in the GSM specifications. Dual carrier will allow the base station to transmit packet data to a mobile on two carriers simultaneously, thereby improving data throughput and reducing delay.
The GSM standard is based on TDMA, where each carrier frequency is allocated 8 timeslots. Each timeslot is allocated to carry either circuit switched voice or packet switched data traffic. A GSM operator is typically allocated a limited number of carrier frequencies by its country spectrum organizer. A common task for the operator is to assign the base stations in the network with frequencies to fulfill capacity (traffic) and coverage needs. Since spectrum is a limited resource, frequency reuse must be employed. The frequency reuse planning and optimization are very complex and time-consuming tasks for operators. In fact, the frequency assignment task is an NP complete problem, which would take impractically long time to solve exactly.
The problem of frequency planning has been researched for many years and there are many suggestions of solutions for this problem in the literature. A promising technique is to use heuristic algorithms for finding a good solution within a reasonable time and with reasonable complexity. Examples of heuristic algorithms are Tabu search, Simulating annealing, Local search, Genetic algorithms and Ant colony optimization. Such algorithms are often utilized in Automatic Frequency Planning (AFP) tools which is a practical approach that has been adopted by many wireless operators for solving the frequency assignment. In the AFP tool, a computer based optimization algorithm, such as one of the above mentioned, is used to find a solution, i.e. a frequency plan, that minimizes an objective function, which typically reflects total network interference. The goal is to minimize interference and maximize quality with a given frequency spectrum.
Further, the interference objective is typically defined by a number of cost functions that model network specifics and other limitations or constraints. The cost functions will give feedback on the implications of the frequency reuse throughout the network and is used by the optimization algorithm to converge to a better solution. In addition, the cost functions are used by the frequency planner to steer the optimizer to a desired solution, i.e. for prioritization of what the planner thinks is most, more or less important for the network. Example of cost functions are:
Interference Cost
A cost that is related to radio interference when frequencies are reused. It can be predicted (from propagation predictions in a planning tool) or measured interference from the system when two sectors reuse co or adjacent frequencies. Typically the interference cost is described by an interference matrix measured in the system see Timner, Y Bergenlid, M. “Estimating the inter cell dependency matrix in a GSM network”, Vehicular Technology Conference, 1999. VTC 1999—Fall. IEEE VTS 50th. However, for new sites or where measurement data is missing, a predicted interference matrix is needed.
Neighbor Cost
Defines a cost if frequencies are reused between sectors defined as neighbors. It could e.g. be used to specify that adjacent frequencies may be used in neighbor cells, while using the same frequencies is forbidden. The neighbor function was introduced to give extra protection for sector neighbors when predicted interference is used since it is hard to predict interference with high accuracy.
Co-Site Cost
Defines a cost if frequencies are reused between sectors (or cells) of a site. It could e.g. be used to specify that adjacent frequencies may be used in a site is allowed while using the same frequency is forbidden. The co-site function was introduced to give extra protection for sectors in a site when predicted interference is used since it is hard to predict interference between sectors at a site accurately.
Co-Sector Cost
Defines a cost if frequencies are reused between transceivers of a sector. It could be that the sector antenna/combiner has requirements of frequency separation between transceivers to work satisfactory. This is mostly handled by the co-sector cost function.
Typically, the frequency planner sets a relative priority between these different functions. This is done by setting a higher cost to the most important task and so on. It is common that co-sector, co-site and neighbor have decreasing priority in order and the interference function has the lowest priority. The optimizer will then try to solve the most important costs first by making a trade-off for lower less important costs. With the setup above, it is beneficial to solve a neighbor cost (i.e. reuse between neighbors) before solving reuse between non-neighbors (i.e. for an interference cost between two other sectors) and so on. In this way, the solution will converge to a lower cost, i.e. less interference.
In practice, the frequency planner may have to repeat optimization runs with different priority settings after doing an analysis of the frequency plan to find the best trade off between the priorities and cost for each function. It may be that the optimizer might not solve all costs (since it is NP complete). This is inherent in the frequency assignment problem.
In the EDGE dual carrier solution, there will be a common LQC (link adaptation/hybrid ARQ) algorithm for both carriers, assigned to an EDGE mobile. As a result, the same MCS will be selected for both carriers, based on the quality information from both carriers assigned to the mobile for EDGE transmissions. Minimal changes of the GSM standard were desired and this was the best solution, considering both performance and cost. Further, this means that the carriers to be combined as dual carriers should be as equal as possible regarding the transmission characteristics, i.e. level of interference, in order to make it possible to select an optimal MCS that may work satisfactorily with both carriers simultaneously.
This may be a problem if the interference is higher on one of the carriers. In this case, it may (in practice) be impossible to transmit data using the same MCS on both carriers. A single carrier must then be used with reduced throughput or increased delay as a result. Thus, the dual carrier technique could not be fully utilized.
Furthermore, with current automatic frequency assignment procedures, it is not possible to assign frequencies for multi carrier operation considering the above requirement with equal interference on the multi carriers. State-of-the-art optimization procedures will make it possible to avoid reuse of frequencies in sectors and put requirements on the frequency separation between carriers of a sector. It is, however, not possible to put requirements on the level of interference between carrier pairs of the same sector which is needed for multi carrier operation. As a result, the optimized frequency plans may not provide that the new multi carrier transmission technique for improved throughput and reduced delay could be offered with a high probability in the network. Instead, it could be that multi carrier connections are downgraded to single carrier connections to a large extent although it is undesired.
Hence, there is a problem of how to generate frequency plans that take into account multi-carrier transmission techniques and which do not suffer from the problems outlined above.